1. Field of the Invention
The present invention relates to a method for producing a resin-bonded magnet and a method for producing an article comprising said resin-bonded magnet. More particularly, the present invention relates to a method for producing a resin-bonded magnet comprising a melt-spin Fe-B-R type magnetic element in which R is Nd and/or Pr and an epoxy resin composition, and a method for producing an article comprising said resin-bonded magnet such as a rotor member of a so-called permanent magnet motor.
2. Description of the Related Art
The resin-bonded magnet is usually formed into a ring-form article such as a ring, a cylinder or a C-shape article, and a supporting member for such molded article also has a ring form or a hollow or solid cylinder form although the supporting member optionally has an engaging element.
It is, however, very difficult to form a ring-form article from the sintered magnet which comprises a rare earth element and cobalt and has, for example, a composition of EQU RCo.sub.5 or R(Co, Cu, Fe, M).sub.n
wherein R is is a rare earth element such as Sm and Ce, M is at least one element selected from the group consisting of the elements of the IV, V, VI and VII groups in the Periodic Table, and n is an integer of 5 to 9, and to impart magnetic anisotropy in a radial direction of said ring-form article. One of the major reasons for this difficulty resides in the fact that coefficients of expansion vary due to anisotropy during sintering. Particularly, when a thin wall ring magnet is to be produced, this drawback should be avoided through isotropy of magnetic properties. Thus, the magnetic characteristic of the sintered rare earth element-cobalt magnet decreases from its inherent value of 20 to 30 MGOe to only about 5 MGOe. In case of the rotor member of the permanent magnet motor which requires high dimensional accuracy, ground finish of the formed rotor member is necessary, which results in a poor yield. In view of such poor yield and use of expensive Sm and Co as major components, the performance and the production cost of the rotor member are not well balanced. Further, since the sintered magnet is mechanically fragile, a part of the magnet will be peeled off, splashed or transferred to another element. This will influence the maintenance of the function or reliability of the permanent magnet motor.
In case of the ring-form article made of the compound of the rare earth element-cobalt magnet and the resin, since the resin matrix compensates for the difference of coefficients of expansion of the magnet which is made anisotropic in the radial direction, the ring-form magnet article can have the magnetic anisotropy in the radial direction. For example, when the magnetic anisotropy is imparted to the injection-molded resin-bonded magnet comprising the rare earth element-cobalt magnet in the axial direction, a magnet with 8 to 10 MGOe :s obtained. Further, the compound magnet has a 30% smaller density and higher dimensional accuracy and is less fragile than the sintered magnet. Therefore, the resin-bonded magnet is a superior material for producing the ring-form magnet, particularly the thin wall ring-form magnet, as compared to the sintered magnet.
As the compound magnet with high magnetic performances, there is known is a resin-bonded magnet comprising a magnet having the composition of EQU Sm(Co, Cu, Fe, M).sub.n
wherein M and n are the same as defined above bound with 2 to 6% by weight of an epoxy resin composition.
Herein, the epoxy resin composition is intended to mean a composition comprising an epoxy resin and a curing agent which three-dimensionally cross links the epoxy resin.
In some cases, the epoxy resin composition may have large influence on the productivity or the performance of the resin-bonded magnet.
In the context of the present specification, the epoxy resin is intended to mean a compound having at least two oxirane rings of the formula: ##STR1## wherein Y is a polyfunctional halohydrin, which may be a residue formed through a reaction between epichlorohydrin and a polyfunctional phenol. Preferred examples of the polyfunctional phenol are resorcinol and bisphenols which are produced by condensation of phenol with an aldehyde or a ketone. Specific examples of the bisphenols are 2,2'-bis(p-hydroxyphenylpropane) (bisphenol-A), 4,4'-dihydroxybiphenyl, 4,4'-dihydroxybiphenylmethane, 2,2'-dihydroxydiphenyloxide and the like. One of the most general epoxy resins is the compound of the formula: ##STR2## wherein R is a divalent group selected from the group consisting of a saturated alkylene group having 1 to 8 carbon atoms, an oxygen atom and a sulfone group, y is an integer of 0 to 25, and m is 0 or 1.
A typical example of this epoxy resin is a condensation product of epichlorohydrin and bisphenol-A (DGEBA).
Examples of the curing agent for the epoxy resin are, in case of the resin-bonded magnet comprising the magnet element of Sm(Co, Cu, Fe, M).sub.n, azole compounds having an amino group and specifically the imidazoles of the formula: ##STR3## wherein R.sub.11, R.sub.12, R.sub.13 and R.sub.14 are the same or different and each a hydrogen atom, a lower alkyl group, a phenyl group or a lower alkyl-substituted phenyl group, examples of which are imidazole, 2-ethyl-4-methylimidazole, 1-benzyl-2-methylimidazole, 1-methylimidazole, 1,2-dimethylimidazole and the like (cf Japanese Patent Kokai Publication Nos. 37106/1985 and 207302/1985).
The epoxy resin composition comprising the above described epoxy resin (e.g. DGEBA) and the imidazole compound as the curing agent generally gives a cured epoxy resin having good chemical resistance and heat resistance. Therefore, such epoxy resin composition is useful to produce the resin-bonded magnet comprising the epoxy resin composition and the magnet of Sm(Co, Cu, Fe, M).sub.n which compound has improved chemical resistance and further a reduced irreversible coefficient at high temperatures.
However, since the imidazole compounds are solid materials having high melting points of usually higher than 200.degree. C., their mixing with the epoxy resin composition is very difficult, and the duration of use of the epoxy resin composition comprising the imidazole compounds is usually less than a week at room temperature. Further, the epoxy resin composition comprising the epoxy resin and the curing agent is gradually polymerized and cured to form dimers or trimers, whereby viscosity of the composition is increased. Since the viscosity of the epoxy resin composition will be increased also in the mixture of said composition with the magnet, when the resin-bonded magnet comprising the magnet of Sm(Co, Cu, Fe, M).sub.n with magnetic anisotropy is to be produced, a degree of magnetic orientation is decreased due to increase of viscosity and for example a residual magnetic flux density is reduced depending on the viscosity increase. Finally, the mixture forms gel, which makes the production of the resin-bonded magnet difficult.
When it is necessary to impart the magnetic anisotropy to the magnet such as Sm(Co, Cu, Fe, M).sub.n, usually the epoxy resin composition is in the liquid form at room temperature so as to make the magnetic orientation easy. However, since the liquid epoxy resin composition has no flowability suitable for compression molding, it is extremely difficult to prepare a green compact for the production of the resin-bonded magnet article. In addition, since the green compact has very low mechanical strength, it is difficult to obtain the resin-bonded magnet article having good quality and performance.
To overcome such defects of the epoxy resin composition, many proposals and suggestions have been made in relation with properties of a binder to be used in the preparation of the green compact. For example, Japanese Patent Kokai Publication No. 63808/1980 discloses the preparation of the green compact by dry blending fine powder of a thermally polymerizable resin and the magnet element. Although this method has some effects on the maintenance of the property of the green compact, a large amount of the binder is required to homogeneously wet the surface of each magnet particle with the thermally polymerizable resin fine powder. As the result, the content of the magnetic element decreases and then the magnetic performance of the magnetic is deteriorated. Japanese Patent Kokai Publication No. 194509/1985 discloses a method for producing a resin-bonded magnet article comprising coating the magnetic element particles with a thermally polymerizable resin composition which is in the solid state at room temperature, molding the green compact at a temperature higher than the softening point of the thermally polymerizable resin and releasing the molded article from the mold at a temperature lower than said softening point. Although this method has some effects on the maintenance of property and performance of the green compact, during molding, the green compact should be heated and cooled in relation to the softening point of the resin. This will create difficult problems in maintenance of production facilities for technical mass production of the resin-bonded magnet articles.
Since some salts of the imidazole compounds with lower fatty acids or phosphoric acid are liquids or solids with lower melting points, the use of such salt will facilitate blending of the epoxy resin with the imidazole compounds. Since the use of such salt will extend the duration of use of the epoxy resin composition, the production of the resin-bonded magnet article which requires the achievement of magnetic anisotropy is stabilized. However, the reaction of the salt of imidazole compound with the epoxy resin produces the monofunctional epoxy resin since the acid residue of said salt reacts with the epoxy resin. Thereby, the crosslink density is decreased so that the chemical and heat resistances of the epoxy-resin composition are deteriorated. The stability of the imidazole salt in which the basicity is neutralized with the acid is improved as the acidity of the acid becomes larger. But, this will further deteriorate the chemical and heat resistances of the epoxy resin composition. That is, the advantageous characteristics of the resin-bonded magnet comprising the imidazole compounds such as chemical resistance and suppression of the irreversible coefficient are deteriorated.
It is possible to produce, for example, a ring-form article from the magnetic element Sm(Co, Cu, Fe, M).sub.n and to impart magnetic anisotropy in the radial direction. For example the method for imparting the magnetic anisotropy to the ring-form article in the radial direction disclosed in Japanese Patent Kokai Publication No. 170501 uses a mold in which magnetic yokes and non-magnetic yokes are alternately arranged around the mold cavity and a magnetizing coil is provided outside the mold or the magnetizing coil is embedded in a mold wall surrounding the cavity. In this method, to generate predetermined strength of magnetic field, a power source with low voltage and large current is used and a magnetomotive force is made large. To effectively focus a magnetic flux excited from outside the mold with the magnetizing coil through the yokes, a magnetic path should be made longer. Particularly, in case of a small ring-form magnet, a large portion of the magnetomotive force is consumed as leakage flux. The magnetic anisotropy in the radial direction cannot be sufficiently imparted to some ring articles having a certain shape. In addition, because of volumetric shrinkage caused by solidification of melt resin-bonded magnet comprising the rare earth element-cobalt magnet element during molding, the ring-form magnet with a thin wall may be cracked. In view of the above described difficulties such as maintenance of the magnetic property in the radial direction or volumetric shrinkage due to solidification of the melt, even the rare earth element-cobalt magnet element has poor shape flexibility in the production of the ring magnet. This will have large influence on design philosophy of the permanent magnet motor including the performance and structure. When the ring-form magnet is used as a rotor member of the permanent magnet motor, the ring-form magnet is often fixed to a shaft through a support made of a material which varies from non-magnetic to magnetic, rather than the ring-form magnet being directly fixed to the shaft. In the former case, the ring-form magnet and the support are mechanically engaged or bonded with an adhesive which is supplied in a narrow gap between them. This is because, in case of the ring-form magnet which requires the generation of magnetic anisotropy in the radial direction, it is very difficult to form the support of a material which varies from non-magnetic to magnetic integral with the ring-form magnet because of limitations on the mold structure with which the ring-form magnet is molded. When the ring-form magnet and the support are mechanically engaged, the ring-form magnet tends to be damaged. When they are bonded with the adhesive, since the gap in which the adhesive is supplied should be present between the ring magnet and the support, bonding strength and dimensional accuracy between the ring-form magnet and the support are not sufficient or the workability is not good. Thus, improvement of assembling property is desired.